C++ in a Nutshell: all you need to know to use ROOT

• cout: the name of the stream linked to the display of the machine. The operator to "feed" cout is the <<.
• The carriage return can be part of the string coded as a \n or expressed as endl.
• The C++ type for strings is the string while you can express string constants enclosed between double quotes (e.g. "My String").

#include <iostream>

using namespace std;

void helloWorld(){
  string helloWorld ("This is an Hello World program!");
  cout << helloWorld << endl;
  
}

• This is a use-case for an uncountable loop steered by the while control structure

#include <iostream>
using namespace std;
void sumMultiples() {
  unsigned int index=0;
  unsigned int addedNumberIndex=0;
  unsigned int sum = 0;
  while (sum<=1000){
    if (index%3 == 0){
      addedNumberIndex++;
      std::cout << addedNumberIndex << ") "<< index << " being added to the sum.\n";
      sum+=index;
    }
    index++;
  }
}

• One way in which you can loop over a container like the vector is to use a running integer index in a for loop
• You can access the elements of the vector using the operator []

#include <iostream>
#include <vector>

using namespace std;

void calculateMomenta () {
  
  unsigned int N=8;
  
  vector<float> myvector;
  myvector.reserve(N);
  
  // fill the vector
  myvector.push_back(1.2f);
  myvector.push_back(11.2f);
  myvector.push_back(1.32f);
  myvector.push_back(0.2f);
  myvector.push_back(1.906f);
  myvector.push_back(3.f);
  myvector.push_back(32.f);
  myvector.push_back(13.f);
  
  
  float sum=0;
  for (unsigned int index=0;index<N;++index){
    sum+=myvector[index];
  }
  
  float mean = sum/N;

  cout << "The mean is: " << mean << endl;  
  
  float RMS2=0;
  for (unsigned int index=0;index<N;++index){
    float element=myvector[index];
    float residual = element - mean;
    RMS2+=residual*residual;
  }
  
  RMS2/=(N-1);
  float RMS=sqrt(RMS2);
  
  cout << "The RMS is: " << RMS << endl;  
}

• By default, C++ passes arguments to functions by value. It means that the parameter is "cloned" within the scope of the function starting from the one passed by the caller. It is therefore pointless to operate changes on "clones" as they will disappear as soon as the function terminates its execution.
• A possible treatment of the parameters given the aforementioned behaviour is to pass values by reference (the signature of the function should be type functionName(type& parameterName))

#include <iostream>
#include <vector>

using namespace std;

void calculateMomenta (vector<float>& myvector, float& mean, float& rms) {

  const unsigned int N = myvector.size();

  float sum=0;
  for (unsigned int index=0;index<N;++index){
    sum+=myvector[index];
  }
  mean = sum/N;

  float rms2=0;
  for (unsigned int index=0;index<N;++index){
    float element=myvector[index];
    float residual = element - mean;
    rms2+=residual*residual;
  }

  rms2/=(N-1);
  rms=sqrt(rms2);

}

//---------------------

void  functions(){

  unsigned int N=8;

  float thisMean, thisRMS;

  vector<float> myvector;
  myvector.reserve(N);

  calculateMomenta(myvector, thisMean, thisRMS);
  std::cout << "Mean: " << thisMean << " --  RMS: " << thisRMS << endl;

  // fill the vector
  myvector.push_back(1.2f);
  myvector.push_back(11.2f);
  myvector.push_back(1.32f);
  myvector.push_back(0.2f);

  calculateMomenta(myvector, thisMean, thisRMS);
  std::cout << "Mean: " << thisMean << " --  RMS: " << thisRMS << endl;

  myvector.push_back(1.906f);
  myvector.push_back(3.f);
  myvector.push_back(32.f);
  myvector.push_back(13.f);

  calculateMomenta(myvector, thisMean, thisRMS);
  std::cout << "Mean: " << thisMean << " --  RMS: " << thisRMS << endl;
}

• The method to add a value to the object is addValue while in order to get mean and rms, they are getMean and getRMS respectively

for (unsigned int index=0;index<N;++index){
        sum+=m_values[index];
    }
    return sum/N;
  };

  float getRms(){
    const unsigned int N = m_values.size();
    float rms2=0;
    for (unsigned int index=0;index<N;++index){
        float element=m_values[index];
        float residual = element - getMean();
        rms2+=residual*residual;
    }
    return sqrt(rms2/(N-1));
  };

private:
  std::vector<float> m_values;

};

int main(){

  Measurements myMeasurements;

  myMeasurements.addValue(-7.31);
  myMeasurements.addValue(6.95);
  myMeasurements.addValue(5.28);
  myMeasurements.addValue(-4.90);
  myMeasurements.addValue(-0.09);
  myMeasurements.addValue(-1.01);
  myMeasurements.addValue(3.03);
  myMeasurements.addValue(5.77);
  myMeasurements.addValue(-8.12);
  myMeasurements.addValue(-9.43);

  std::cout << "Mean: " << myMeasurements.getMean() << ", RMS: " << myMeasurements.getRms() << std::endl;
  return 0;
}